1. Field of the Invention
The present invention relates to micro-mechanical devices for reflective displays and, more particularly, to a reflective display having deformable mirrors.
2. Description of the Related Art
There are significant efforts underway to develop low power, high resolution, "paper-like" displays using either liquid crystals (T. Ogawa et al., "The Trends of reflective LCDs for future electronic paper", SID '98 Digest, p. 217) or other technologies. The liquid crystal based approaches generally suffer from low reflectivity and poor contrast ratios. There has recently been a number of publications on using MEM (microelectromechanical) devices for display applications. Projection systems based on arrays of tilting mirrors have been commercialized (See, e.g., J. Sampsell, "An overview of the digital micromirror device (DMD) and its application to projection displays", SID '93 Digest, p. 1012) and projection systems proposed using a micromechanical phase grating (See, e.g., D. Bloom "The grating light valve: revolutionizing display technology", SPIE Vol. 3013 (1997) p. 165).
Two types of MEM based direct view displays have also been proposed. In the first (See e.g., E. Stern, "Large-area micromechanical flat-panel display", SID 97 Digest, p. 230), an array of passively addressed bistable transparent beams are used to control the release of light trapped by total internal reflection. This device uses a back light, and due to the thick optical feed structure (about 4 cm) is not be suitable for portable displays. A second direct view display (See e.g., M. W. Miles, "A new reflective FPD technology using interferrometric modulation", SID 97 Digest (1997) p.71) includes the use of a micromachined deformable optical cavity whose reflected color changes with voltage. The device includes a self-supporting deformable membrane, made of, for example aluminum, and a thin film stack, both residing on a transparent substrate. The self-supporting deformable membrane and the thin film stack act as mirrors for an optically resonant cavity. When a voltage is applied, the deformable mirror collapses and the color of the reflected light is changed. The devices are binary and have hysteresis which allows passive addressing. The color selection of the two states is determined by the optical stack (which contains a conductor) and by the rest height of the deformable mirror. The main disadvantage of such a system is that the maximum reflectivity is limited. For a narrow color band, the peak reflectance can be about 80%. If an 80% reflectivity is assumed for the whole Red, Green, and Blue bands, a triad pixel structure, and an 80% aperture ratio, the maximum white reflectivity would be about 21%. For a paper-like display, a reflectivity of 40% or more is necessary.
A type of display, referred to as "electroscopic displays", have been described by T. S. Te Velde et al. which are bistable and have an improved reflectivity compared to the interferrometric modulation displays described above. (See Te Velde et al., "A family of electroscopic displays", Society of Information Display 1980 technical digest, p.116-117 and the following U.S. Pat. Nos. 4,178,077, 4,519,676, 4,729,636.) The article entitled, "A family of electroscopic displays" (hereinafter Velde), describes an electroscopic fluid display where a plate or grid which is reflective and is movable is sealed with a glass plate and filled with a nonconducting black or other colored solvent. If the penetration depth of the incident light in the solvent is much smaller than the thickness of the cell, than when the white grid is located near the bottom plate, the grid will not be visible and the cell will appear black. However, when the white grid is attracted to the front side, the white grid will be visible and the cell will appear white.
Two possible configurations are described in Velde, a springy capacitor and a triode. For the springy capacitor, the grid is mechanically fastened to the bottom plate via conductive springs and when a large enough voltage is applied, the springs are stretched and the grid rushes to the upper electrode. This arrangement requires careful cell gap control since the threshold voltage is a function of the cell gap.
In the triode configuration, the springs are made very weak so that mechanical forces can be neglected and electrodes on the top and bottom plates are used to electrostatically control the position of the reflective plate. In U.S. Pat. No. 4,178,077, a triode configuration is described where electrostatic forces are used to control a movable electrode in an opaque liquid. A fabrication process is also described which uses an underetching process where apertures in a second layer provide access for the etchant to the first layer. This requires a timed etch to leave portions of the first layer in place to support the second layer. In U.S. Pat. No. 4,519,676, a triode configuration is again described, but with the resilient elements below the display part to increase the aperture ratio. A more complicated fabrication process is described which again uses timed underetching. In U.S. Pat. No. 4,729,636, engaging points are formed between the movable electrode and its engaging surface to improve the response time by allowing liquid flow in and out during closure and release. The triode configuration is complicated and requires electrical contacts for addressing to be formed on both top and bottom plates. Both the triode and springy capacitor (when fastened to the bottom plate) require precise cell gap control since the threshold voltage depends on the cell gap. The fabrication processes described require the etching step to be stopped by a certain time or the first layer will be fully removed and the second layer will no longer be attached to the substrate.
For a high information content display, such as one for an 8.5 inch by 11 inch sized display with 150 dot per inch resolution, it is advantageous to integrate some of the addressing electronics on the display itself to reduce cost and improve yield. For the display size described above, approximately 1,275 gate line and about 1,650 data line connections and driver chip outputs are needed. If the display technology used can also provide switches, the row selection circuits (i.e., shift register) and data driver demultiplexing circuits may be made with the display and greatly reduce the number of connections and drivers. (See "Silicon light valve array chip for high resolution reflective liquid crystal projection displays", by J. L. Sanford et al., IBM J. Res. Develop., Vol. 42 No. 3/4, May/June 1998, pp.347-358, incorporated herein by reference.)
Therefore, a need exists for a portable display having high reflectivity and a high contrast ratio. A further need exists for a display which permits switches to be fabricated at the same time as the display device. A still further need exists for a method for fabricating the display device and switches in an efficient and economical manner.